How to stop the character after walking
First thing, switch from Dope Sheet to Graph Editor (CtrlTab), in the View menu, disable Show Group Colors to make the channels readable :

As we can see if you select the root bone, then select its Y Location channel, you have enabled the Linear Extrapolation on the Y location (ShiftE &gt; Linear Extrapolation), which makes it continue indefinitely its movement on this axis. To stop it, choose ShiftE &gt; Constant Extrapolation instead:

Once done, if you want the character to stop at a certain position on the scene at frame 240, just move it to this position and keyframe.
The fact that some of your character's limbs stretch at the end is because some other bones, like the hands, have been set to Linear Extrapolation as well for several of its location channels, so select all these channels and correct that too :